For various permanent magnet applications, it is known to produce a fully dense rod or bar of a permanent magnet alloy, which is then divided and otherwise fabricated into the desired magnet configuration. It is also known to produce a product of this character by the use magnet particles, which may be prealloyed particles of the desired permanent magnet composition. The particles are produced for example by either casting and comminution of a solid article or gas atomization of a molten alloy. Gas atomized particles are typically comminuted to achieve very fine particle sizes. Ideally the particle sizes should be such that each particle constitutes a single crystal domain. The comminuted particles are consolidated into the essentially fully dense article by die pressing or isostatic pressing followed by high-temperature sintering. To achieve the desired magnetic anisotrophy, the crystal particles are subjected to alignment in a magnetic field prior to the consolidation step.
In permanent magnet alloys, the crystals generally have a direction of optimum magnetization and thus optimum magnetic force. Consequently, during alignment the crystals are oriented in the direction that provides optimum magnetic force in a direction desired for the intended use of the magnet. To provide a magnet having optimum magnetic properties, therefore, magnetic anisotrophy is achieved with the crystals oriented with their direction of optimum magnetization in the desired and selected direction.
This conventional practice is used to produce rare-earth element containing magnet alloys and specifically alloys of neodymium-iron-boron. The conventional practices used for this purpose suffer from various disadvantages. Specifically, during the comminution of the atomized particles large amounts of cold work are introduced that produce crystal defects and oxidation results which lowers the effective rare-earth element content of the alloy. Consequently, rare-earth additions must be used in the melt from which the cast or atomized particles are to be produced or in the powder mixture prior to sintering in an amount in excess of that desired in the final product to compensate for oxidation. Also, the practice is expensive due to the complex and multiple operations prior to and including consolidation, which operations include comminuting, aligning and sintering. The equipment required for this purpose is expensive both from the standpoint of construction and operation.
Permanent magnets made by these practices are known for use with various types of electric motors, holding devices and transducers, including loudspeakers and microphones. For many of these applications, the permanent magnets have a circular cross section constituting a plurality of arc segments comprising a circular permanent magnet assembly. Other cross-sectional shapes, including square, pentagonal and the like may also be used. With magnet assemblies of this type, and particularly those having a circular cross section, the magnet is typically characterized by anisotropic crystal alignment.
During mechanical working the crystals will tend to orient in the direction of easiest crystal flow. This results in mechanical, crystal anisotrophy. The preferred orientation from the standpoint of optimum directional magnetic properties is desirably established in the optimum crystal magnetization direction by this mechanical crystal anisotrophy.